You can also define a specific error log file for each VirtualHost in Apache. If you have any VirtualHost defined in /etc/apache2/sites-available/ and enabled in /etc/apache2/sites-enabled (enable with sudo a2ensite [your-virtualhost-definition-file]), you can change the error log by adding the following line inside your VirtualHost config:

ErrorLog ${APACHE_LOG_DIR}/[your-vhost]-error.log

That might be useful if you have a lot of vhosts and want to split where they report the errors.

Also, you can watch your error log live by issuing the following command (adapt to your own log file if different from the default):